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Welding wire strength in relation to base metals, weld strength

01-27-2011, 02:19 AM

If one is joining two tubes of identical metal, how does on choose the welding wire? Assume I'm building a roll cage out of mild DOM steel. I notch the tube, fit it up, and grab my MIG/TIG welder. Do I choose a wire that has the strength of the tubing, weaker, or stronger? Is it different if I'm butt-joining two tubes, using a tube on the inside to rosette weld in place, then running a couple beads to join the exterior tubes to the interior one? Is the joint, after construction, weaker or stronger than the metals I welded together? Assume I'm a competent welder who has a full-time job assembling roll cages (or airframes, etc); of course, I don't have such a job, and I'm not a professional welder.

If I take two plates (or two sections of tubing) and do a butt joint (assume I grind down the plate edge/tubing edge to allow for full penetration of the weld), then section off a couple coupons, I'm aware of two tests for strength: one is where I bend the coupon to ensure it doesn't fail at the joint, the other is putting the coupon in tension until it fails. My understanding is any coupon failing at the weld is bad; is this wrong? Are there other tests for weld strength that are used?

This comes about because someone I know was told in a class on welding roll cages for NASCAR that the welds will be the weakest part, and that you always use a weaker metal as filler. This seems contrary to what I've read and heard elsewhere, so if one of us is off base, I'd like to know. I assume in the case of a roll cage or airframe, you would take the entire unit and heat treat it to help the HAZ. Is this not the case?

I see the American Welding Society has a publication, D1.1, which probably has all the answers; they want a couple hundred dollars for it, though. If you can point to any online resources which back up your positions, that would be great, too.

there is also the 505 test. This test is typically just for the weld metal. The SOP is to weld a very wide joint where you can isolate the weld metal where the dilution is going to be minimal, you cut the weld metal out and then machine it to the 505 test dimensions. The test then is a tensile test.

The most important tests are the ones that prove your weld is strong enough, yes, failure in the weld metal is failure. You are looking for a base metal failure but you want to be careful to not over weld or over match with too strong of a filler because that will drive your base to failure faster. If the weld is significantly stronger (different for every base metal type) than the bases metal it will put a stress riser in the heat affected zone and it will fail prematurely.

Most of the time for mild steel such as A36 which yields at around 36KSi but reaches a UTI (ultimate tensile) between 58,000 and 79,800 PSi you would try to match the UTI so you would use a wire of 70KSi strength, if your stick welding you could use a 60XX series for the root and a 70XX series for the fill and cover passes (for stick 60XX and 70XX both have benefits and drawbacks but are suited for use together.
Take a look at the link to eagle steel, I use this site because they have an easy format to work with, there are plenty of others out there too.http://www.eaglesteel.com/download/t...eel_Grades.pdf

Comment

Nascar tube frames and aircraft fuselages are unique. That is when you weld them you typically undermatch the filler rod to the base metal (usually 4130N tubing). Most guys prefer to weld 4130 (105-120,000 psi strength, depending on manufacturer) with ER70S-2 (70,000psi) because mixing the 2 different metals gives the weld good characteristics such as good ductility and a reasonable strength match. If you assume to reach approximately 30% dilution with the 70 series filler rod, you are still going to have a weld that is 85-90,000 psi-- close to the base metal strength but still the weak point.

The other reason to use the mild steel wire is to avoid having to heat treat the entire roll cag/fuselage after fabrication. Biggest reason for this is cost. The following 2 articles will give you more of the technical stuff particular to normalized 4130 welding:

Comment

No worries, glad I could help (if I did in fact help).
Finding an oven big enough to put a 20 plus foot long fuselage in is impossible, so we opt for a slightly lower yield strenth along with greater ductility in the weld in order to avoid post-weld heat treating

Comment

One pipeliners have to also pass per the API 1104 is a "Nick Break Test". The test coupon is intentionally weakened in the weldment/filler metal zone by nicking it (usually with a hack saw) and then destructively testing. It's somewhat like a tensile strength test, but the intent is to get a visual of the cross section of the weldment to look for flaws such as slag inclusions, gas pockets, etc.